Method and system for controlling power distribution in a hybrid fuel cell vehicle

ABSTRACT

A power distribution control system for a hybrid fuel cell vehicle is provided. The system includes a high energy converter (HEC) for providing current from an electrical bus to the battery, or from the battery to the bus. As a vehicle load changes, the HEC adjusts the battery current flow. A current command is sent to a fuel cell controller so that the fuel cell current output is adjusted. As the fuel cell current output changes, the HEC further adjusts the battery current flow, until there is a zero net current flow to and from the battery. At this point, a state of equilibrium is reached and the fuel cell provides all the current required by the vehicle loads.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention relates to a method and system for controllingpower distribution in a hybrid fuel cell vehicle.

2. Background Art

A hybrid fuel cell vehicle may include three power sources for itselectrical loads: a battery, the fuel cell, and a traction motor. Whilepowering the vehicle, the traction motor is a load, but duringcoast-down, the traction motor becomes a generator. This regenerativepower can supply current to the other loads or be used to charge thebattery. Coordinating the current flow between these sources and thecontinually varying electrical loads presents a fundamental controlproblem. Imprecise control can result in reduced fuel economy, poorperformance, reliability problems, and possible electrical businstabilities.

In addition, there are other considerations when utilizing a fuel cellin a power distribution system. For example, unlike a battery, a fuelcell may not be able to instantly supply sufficient current to meet theneeds of an increased electrical load.

Therefore, the battery needs to “fill in” current temporarily, thentaper off when the fuel cell's current output increases. Without thepresence of the battery to temporarily supply current, performance maydegrade. In addition, the battery may also provide a repository forexcess fuel cell current and regenerative current during braking andcoast down.

One attempt to integrate a battery into a hybrid fuel cell vehicle isdescribed in SAE Paper No. 2002-01-0096, titled “Development ofFuel-Cell Hybrid Vehicle” (“the SAE Paper”). The SAE Paper describes theuse of a battery connected in parallel with fuel cells via a DC/DCconverter. The battery is configured to provide a power assist when fuelcell response is delayed, or when the vehicle is driven under high loadconditions. The traction motor is located between the fuel cell and theconverter; whereas, the fuel cell auxiliary systems are located betweenthe battery and the converter. To determine the fuel cell operationalpoint, power-current (P-I) and current-voltage (I-V) maps are used. Apower requirement is input, and using the P-I and I-V maps, a voltagecommand is determined.

One limitation of the hybrid vehicle described in the SAE Paper is itsuse of operating modes which do not utilize the fuel cell, but rather,rely solely on the battery to supply all of the power. In such operatingmodes, all of the vehicle electrical loads are carried by the battery.This may require the use of an undesirably large battery, or placelimits on the loads the system is able to handle.

In addition, the SAE Paper does not describe a system or method forcontrolling the rate of change of current flow to or from the battery,nor does It describe how to determine a target rate.

Accordingly, there exists a need for a method and system that providefor controlling power distribution in a hybrid fuel cell vehicle suchthat a fuel cell works in conjunction with a second power source, suchas a battery, ultra-capacitor, or other equivalent electrical storagedevice, to provide power to vehicle electrical loads, and a systemequilibrium is sought, wherein the fuel cell carries all of the vehicleelectrical loads, and the current flow of the second power source isadjusted at least partly based on a measured voltage, and at apredetermined rate, until a predetermined constant is reached.

SUMMARY OF INVENTION

Therefore, a power distribution control system for a vehicle having afuel cell and a second power source connected to an electrical bus isprovided. The control system includes a voltage regulator configured tocontrol voltage on the bus. A first controller controls the voltageregulator. A computer is programmed and configured with fuel cellcharacteristics for relating fuel cell voltage to fuel cell current. Thecomputer is further programmed and configured to receive a currentrequest at least partly based on vehicle loads, and to determine a firstvoltage related to the current request using the fuel cellcharacteristics. A second controller is configured to receive a voltagesignal from the computer and to provide a current command to the firstcontroller. The voltage signal is at least partly based on the firstvoltage and a measured voltage.

Some embodiments of the invention also include a power distributioncontrol system having electrical loads connected directly to the fuelcell, which provides a low cost, efficient architecture. Since mainpower current can go directly from the fuel cell to the loads withoutpassing through another device, the battery and voltage regulator sizecan be minimal. This may result in an overall cost savings.

In addition, embodiments of the invention may utilize a single voltagesensor to measure a voltage on the electrical bus to help control thevoltage regulator. Because voltage sensors are often used to providevoltage measurements to other vehicle systems, a separate voltage sensormay not be needed in the present invention. Moreover, the use of asingle voltage sensor, rather than multiple sensors, may provide anoverall cost savings.

The invention also provides a method of controlling the powerdistribution in a vehicle having a fuel cell and a second power source.The method includes generating a first voltage based on a vehicleelectrical load change. A first current command is generated at leastpartly based on the first voltage and a measured voltage. Current flowof the second power source is adjusted at least partly based on thefirst current command, and the available fuel cell current is adjustedat least partly based on the vehicle electrical load change. The currentflow of the second power source is continuously adjusted at least partlybased on additional current commands until an equilibrium point isreached.

The invention further provides a vehicle having a fuel cell and a secondpower source connected to an electrical bus, and a power distributionsystem for controlling the distribution of power in the vehicle. Thepower distribution system includes a voltage regulator configured tocontrol the voltage on the bus. A first controller controls the voltageregulator, and a computer is programmed and configured with fuel cellcharacteristics for relating fuel cell voltage to fuel cell current. Thecomputer is further programmed and configured to receive a currentrequest at least partly based on vehicle loads. The computer is alsoprogrammed and configured to determine a first voltage related to thecurrent request using the fuel cell characteristics. A second controlleris configured to receive a voltage signal from the computer, and toprovide a current command to the first controller. The voltage signal isat least partly based on the first voltage signal and a measuredvoltage.

The invention also provides a controller for controlling the powerdistribution in a vehicle having a fuel cell and a second power source.The controller includes an algorithm for generating a first voltage atleast partly based on vehicle electrical loads, for generating a currentcommand at least partly based on the first voltage and a measuredvoltage, for adjusting current flow of the second power source at leastpartly based on the current command, for adjusting available fuel cellcurrent at least partly based on the vehicle electrical loads, and forcontinuously adjusting the current flow of the second power source untilan equilibrium point is reached.

The above object and other objects, features, and advantages of thepresent invention are readily apparent from the following detaileddescription of the best modes for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a control diagram illustrating the system of the presentinvention;

FIG. 2 shows a family of V-I polarization curves for a fuel cell; and

FIG. 3 is a graph illustrating the relationship between bus voltage,vehicle electrical loads, available fuel cell current, and batterycurrent flow.

DETAILED DESCRIPTION

FIG. 1 is a control diagram illustrating a system 10 in accordance withthe present invention. In this embodiment, the system 10 is part of ahybrid fuel cell vehicle (not shown). A fuel cell 12 is connected to ahigh voltage bus 14, and supplies current directly to all the electricalloads. The fuel cell 12 may be considered a fuel cell subsystem,including both a fuel cell stack, and a fuel cell auxiliary system, suchas an air compressor and/or a deionized heater. The electrical loads asshown in FIG. 1 include an electric motor 16 and all other electricalloads 18.

The electrical loads 18 may include many different types of loads. Forexample, the electrical loads 18 may include an inverter for receivingdirect current from the bus 14, and for providing alternating current tothe motor 16. Other types of electrical loads include heating,ventilating, and air conditioning (HVAC) systems, water pumps, fans, apower steering pump, and various low voltage systems for runningelectronic components within the vehicle. It is understood thatdifferent vehicles will have different components connected to the bus14.

A second power source, or battery 20, is connected to the bus 14 througha voltage regulator, or high energy converter 22 (HEC). The term“second” power source merely implies a power source in addition to thefuel cell 12. Although the second power source in this embodiment is abattery, and in particular a nickel metal hydride battery, it could bevirtually any type of power source capable of supplying and receivingcurrent. Other examples of a second power source include a lead acidbattery and an ultra-capacitor.

The HEC 22 is a bidirectional buck-boost voltage regulator that cansource current onto the bus 14 from the battery 20, or it can takecurrent off the bus 14 and place it into the battery 20. The latterfunction is commonly referred to as regeneration. The HEC 22 is referredto as a “buck-boost” voltage regulator because it has the ability toincrease, or boost, the voltage of the battery 20 to match the voltageof the bus 14, or alternatively, reduce, or buck, the voltage of thebattery 20 to match the voltage of the bus 14.

The system 10 includes a first controller, or HEC controller 24, and acomputer, or vehicle system controller (VSC) 26. The VSC 26 isresponsible for the overall control and monitoring of the vehicle.Although shown in FIG. 1 as separate components, at least a portion ofthe HEC controller—e.g., a digital portion—may be integrated into theVSC 26. That is, the HEC controller 24 may be a software controller,part of a control algorithm that the VSC 26 is programmed and configuredto execute. Of course, a HEC controller, such as the HEC controller 24,may be a physical device separate from the VSC 26, or it may beintegrated into the software of another controller.

The HEC 22 is a primary control device for maintaining the balance ofcurrents on the bus 14. The HEC 22 acts as a bidirectional gateway forcontrolling battery current flow. On the high voltage load side of thebus 14, the HEC 22 acts like a voltage regulator. As described morefully below, the HEC controller 24 receives a current command (I CMD)from a second, or closed loop controller (CLC) 28. The HEC controller 24then signals the HEC 22, which adjusts the battery current flow. Ofcourse, the CLC 28 may send many current commands to the HEC controller24 for continuously adjusting the battery current flow as a systemequilibrium is sought. Like the HEC controller 24, the CLC 28 may be asoftware controller, for example, part of a control algorithm that theVSC 26 is programmed and configured to execute. Alternatively, the CLC28 may be a separate physical device, or integrated into the software ofanother controller.

A third controller, or fuel cell controller 30, is also part of thesystem 10. It also may be separate from, or integrated into, the VSC 26.The fuel cell controller 30 is configured to send signals to, andreceive signals from, the VSC 26. For example, the VSC 26 may send acurrent request (I REQ) to the fuel cell controller 30, so that the fuelcell controller 30 will adjust the available fuel cell current inresponse to a vehicle electrical load change.

In the embodiment shown in FIG. 1, all of the vehicle electrical loads16, 18 are connected to the bus 14 between the HEC 22 and the fuel cell12. This is not required for the control system 10 to function; however,such a configuration may have certain advantages. For example, when thevehicle electrical loads are directly connected to the fuel cell 12, asthey are in this configuration, the current can flow from the fuel cell12 to the loads without passing through another device. This is aninherently efficient architecture. In addition, such an architecture maybe relatively low in cost, since the battery 20 and HEC 22 need not belarge, as they only temporarily fill in current when the fuel cell 12cannot immediately provide it.

When a change in any of the vehicle loads first occurs, it is detectedby the VSC 26. For example, if a vehicle occupant initiates a requestfor cool air from a vehicle air conditioning system, and an airconditioning compressor is started, the air conditioning compressorwould then signal the VSC 26 with a current request (I REQ). The VSC 26would, in turn, send the current request to the fuel cell controller 30.The fuel cell controller 30 would then adjust (in this example,increase) the available fuel cell current to provide the additionalcurrent required by the air conditioning compressor.

Although the fuel cell 12 may ultimately provide all the currentrequired to maintain all of the vehicle electrical loads, it may not bepossible for it to immediately provide the additional current requiredwhen loads suddenly increase. In such situations, the battery 20 mayprovide the necessary current more quickly than the fuel cell 12,thereby helping to ensure that the system 10 can respond quickly toelectrical load changes. Thus, as illustrated In FIG. 1, the VSC 26 notonly communicates the current request (I REQ) to the fuel cellcontroller 30, but also applies it to certain fuel cell characteristics,in this embodiment a V-I polarization curve 32.

A polarization curve is a voltage versus current plot that describes thestable equilibrium points of devices such as fuel cells and batteries.Polarization curves change not only from one fuel cell to another, butalso change within a single fuel cell for different operating states.Thus, the relationship between voltage and current in a fuel cell is notadequately described by a single polarization curve. This is because thepolarization curve is dependent upon the temperature of the fuel celland other parameters.

FIG. 2 shows a family of polarization curves for a fuel cell, such asthe fuel cell 12, shown in FIG. 1. It is readily seen that there is agenerally inverse relationship between voltage and current—i.e., as oneincreases, the other decreases. Moreover, the curves themselves changeas the fuel cell temperature changes. Thus, in order to efficiently usea polarization curve for a given fuel cell, a family of curves may beused that define the voltage/current relationship over the operatingtemperatures of the fuel cell.

Returning to FIG. 1, the V-I polarization curve 32 represents a familyof curves, the characteristics of which are programmed into the VSC 32.Of course, fuel cell characteristics, such as V-I polarization curves,may be programmed into a separate computer that is in electricalcommunication with the VSC 26. Applying the requested current (I REQ) tothe V-I polarization curve 32 yields a first voltage, or referencevoltage (V REF). The reference voltage (V REF) is summed with a measuredvoltage (V MEAS) at a summing junction 34. This yields a voltage error(V ERR) that is ultimately sent to the CLC 28.

It is the voltage error (V ERR) that the CLC 28 uses to generate thecurrent command (I CMC) that is sent to the HEC controller 24. By usingthe V-I polarization curve 32 to generate the reference voltage (V REF),the system 10 can be monitored using the measured voltage (V MEAS)instead of a measured current. Because a voltage sensor will be presentin most control systems, such as the system 10, the need to add currentsensors is obviated. In addition, the measured voltage (V MEAS) can bepicked up in one location on the bus 14; whereas, if currentmeasurements are used, it may be necessary to take them in multiplelocations. Thus, an overall cost savings may be realized by using acontrol system, such as the system 10, that utilizes a single voltagemeasurement.

As seen in FIG. 1, the voltage error (V ERR) may pickup some disturbancevoltage (V DIST) prior to reaching the CLC 28. The disturbance voltagemay result from system noise, caused by any one, or a combination, ofdifferent electrical loads connected to the bus 14. The CLC 28, whichmay be a proportional plus integral plus derivative controller, receivesthe voltage signal from the VSC 32, and calculates a current command (ICMD), which it sends to the HEC controller 24.

In this way, the HEC 22 can be commanded to adjust the battery currentflow to compensate for a vehicle electrical load change. Thus, thebattery 20 may respond to a load change by providing more current to thebus 14, such as when a load increases, or it may respond by takingcurrent from the bus 14, such as when a load quickly drops. Of course,as the HEC 22 is adjusting the battery current flow to compensate forload changes, the VSC 26, in conjunction with the fuel cell controller30, is adjusting the available fuel cell current.

A goal of the system 10 is to reach an equilibrium state, such that allof the current required by the vehicle electrical loads is provided bythe fuel cell 12. Thus, after initially adjusting the battery currentflow based on an initial current command (I CMD), the VSC 28 providesadditional current commands to bring the battery current flow to apredetermined constant. The predetermined constant may be zero, or maybe another value, and can be programmed into the VSC 26. The additionalcurrent commands may be based on how long it will take the fuel cell 12to compensate for the change in the vehicle electrical loads. This ratemay be a predetermined rate, based on known characteristics of the fuelcell 12. The predetermined rate may be provided by a fuel cellmanufacturer, or may be empirically generated for a given fuel cell, orclass of fuel cells.

FIG. 3 illustrates the relationship over time (t) between the busvoltage (V BUS), the vehicle loads (I LOAD), the fuel cell currentavailable (I FC AVAIL), and the battery current flow (I HEC) when avehicle load changes. For clarity, all the vehicle electrical loads arerepresented by (I LOAD). Thus, the sum of the available fuel cellcurrent (I FC AVAIL) and the battery current flow (I HEC) equals thevehicle electrical load (I LOAD). Initially, the system is atequilibrium (Time A). During this time, all of the vehicle current loadis being provided by the fuel cell (I LOAD =I FC AVAIL). At the end ofTime A, a vehicle load increases. As seen in FIG. 3, the available fuelcell current does not initially change. This may be due, in part, to theslow response time characteristic of fuel cells.

The battery current (I HEC) quickly changes in response to the vehicleload change. With reference to FIG. 1, the vehicle load increase wouldbe detected by the VSC 26, which may receive a current request (I REQ)directly from a module, such as an air conditioning compressor. The VSC26 then applies the current request (I REQ) to the V-I polarizationcurve 32 to generate the reference voltage (V REF). The measured voltage(V MEAS) is picked up on the bus 14 and subtracted from the referencevoltage to generate the voltage error (V ERR).

The voltage error may pickup a disturbance voltage (V DIST) prior tobeing input into the CLC 28. The CLC 28 generates a current command (ICMD), which is sent to the HEC controller 24. The HEC controller 24 thensignals the HEC 22 to increase the battery current flow to compensatefor the load change. The change in battery current flow is seen in FIG.3, occurring during (Time B). During Time B, the battery current flowmay receive additional adjustments based on the measured voltage, sothat the increase in battery current flow tracks the load increase.

As discussed above, VSC 26 will also send the current request (I REQ) tothe fuel cell controller 30. Because the load change in this example isa load increase, an increase in current will be requested from the fuelcell 12. At the end of Time B, the fuel cell 12 begins to generateadditional current. This is illustrated in FIG. 3 by the rising (I FCAVAIL) curve.

During Time C, the available fuel cell current continues to increasebased on the fuel cell current requested by the fuel cell controller 30.Also during Time C, the battery current flow is continuously adjusted,in this example decreased, in conjunction with the increase in availablefuel cell current. The battery current flow is decreased based on apredetermined rate of change of available fuel cell current (I FCAVAIL). As discussed above, the predetermined rate can be programmedinto the VSC 26, such that the VSC 26, in conjunction with the CLC 28,generates additional current commands to continuously adjust the batterycurrent flow until equilibrium is reached. Finally, at the end of TimeC, the available fuel cell current is equal to the vehicle electricalload (I LOAD), and the battery current flow is zero. Thus, the system isin equilibrium, where it remains during Time D until another load changeoccurs.

Although the discussion above Involved a load increase, a similarsituation occurs in the case of a load decrease. Specifically, adecrease in current load would result in a voltage increase on the bus14, and the reduced current load request would be detected by the VSC26. The battery current flow would then be decreased to compensate forthis change. In fact, the HEC 22 may direct current flow into thebattery 20, thereby facilitating regeneration. In addition, the motor 16may be configured to use excess current in the event of a sharp loaddecrease. The fuel cell current requested would then decrease over time,and the battery current flow would be adjusted in conjunction with thisdecrease. The system would again reach equilibrium, and maintain asteady-state condition until another vehicle electrical load change.

While the best mode for carrying out the invention has been described indetail, those familiar with the art to which this invention relates willrecognize various alternative designs and embodiments for practicing theinvention as defined by the following claims.

What is claimed is:
 1. A power distribution control system for a vehiclehaving a fuel cell and a second power source connected to an electricalbus, the control system comprising: a voltage regulator configured tocontrol voltage on the bus; a first controller for controlling thevoltage regulator; a computer programmed and configured with fuel cellcharacteristics for relating fuel cell voltage to fuel cell current, thecomputer being further programmed and configured to receive a currentrequest at least partly based on vehicle loads, and to determine a firstvoltage related to the current request using the fuel cellcharacteristics; and a second controller configured to receive a voltagesignal from the computer and to provide a current command to the firstcontroller, the voltage signal being at least partly based on the firstvoltage and a measured voltage.
 2. The control system of claim 1,wherein the second power source is one of a nickel metal hydridebattery, a lead acid battery, and an ultra-capacitor.
 3. The controlsystem of claim 1, further comprising a third controller for controllingthe fuel cell.
 4. The control system of claim 1, wherein the computer isfurther programmed and configured to execute control algorithms, thecontrol algorithms including the first controller, the secondcontroller, and the third controller.
 5. The control system of claim 1,wherein the voltage regulator and the fuel cell are disposed on the bussuch that all vehicle electrical loads connect to the bus between thevoltage regulator and the fuel cell.
 6. The control system of claim 1,wherein the fuel cell characteristics comprise a plurality of fuel cellpolarization curves.
 7. The control system of claim 1, wherein thevoltage signal received by the second controller is a voltage error, thevoltage error being the difference between the first voltage and ameasured voltage.
 8. The control system of claim 1, wherein the secondcontroller is further configured to provide current commands to thefirst controller to achieve a target current flow for the second powersource.
 9. The control system of claim 7, wherein the target currentflow for the second power source is a predetermined constant.
 10. Amethod of controlling the power distribution in a vehicle having a fuelcell and a second power source, the method comprising: generating afirst voltage based on a vehicle electrical load change; generating afirst current command at least partly based on the first voltage and ameasured voltage; adjusting current flow of the second power source atleast partly based on the current command; adjusting available fuel cellcurrent at least partly based on the vehicle electrical load change;continuously adjusting current flow of the second power source at leastpartly based on additional current commands until an equilibrium pointis reached.
 11. The method of claim 9, wherein the equilibrium point isreached when the current flow of the second power source is apredetermined constant.
 12. The method of claim 9, wherein the firstcurrent command is generated at least partly based on a voltage error,the voltage error being the difference between the first voltage and themeasured voltage.
 13. The method of claim 9, wherein the continuousadjustment of the current flow occurs at a predetermined rate, thepredetermined rate being based on calibrated fuel cell current output.14. The method of claim 9, wherein the first voltage is generated byapplying a current request to a fuel cell polarization curve.
 15. Themethod of claim 14, wherein the fuel cell polarization curve is chosenfrom a family of curves based on fuel cell operating conditions.
 16. Themethod of claim 9, wherein the current command is partly based on adisturbance voltage.
 17. A vehicle having a fuel cell and a second powersource connected to an electrical bus, and a power distribution systemfor controlling the distribution of power in the vehicle, the powerdistribution system comprising: a voltage regulator configured tocontrol voltage on the bus; a first controller for controlling thevoltage regulator; a computer programmed and configured with fuel cellcharacteristics for relating fuel cell voltage to fuel cell current, thecomputer being further programmed and configured to receive a currentrequest at least partly based on vehicle loads, and to determine a firstvoltage related to the current request using the fuel cellcharacteristics; and a second controller configured to receive a voltagesignal from the computer and to provide a current command to the firstcontroller, the voltage signal being at least partly based on the firstvoltage and a measured voltage.
 18. The vehicle of claim 17, wherein thefuel cell characteristics comprise a plurality of fuel cell polarizationcurves.
 19. A controller for controlling the power distribution systemin a vehicle, the vehicle having a fuel cell and a second power source,the controller comprising: an algorithm for generating a first voltageat least partly based on vehicle electrical loads, for generating acurrent command at least partly based on the first voltage and ameasured voltage, for adjusting current flow of the second power sourceat least partly based on the current command, for adjusting availablefuel cell current at least partly based on the vehicle electrical loads,and for continuously adjusting the current flow of the second powersource until an equilibrium point is reached.
 20. The controller ofclaim 19, wherein the equilibrium point is reached when the current flowof the second power source is a predetermined constant.